The present invention relates to a device for detecting the concentration of oxygen in automotive exhaust gas. More particularly, the invention relates to a device for detecting the concentration of oxygen in a lead-containing exhaust gas.
A conventional device for detecting the concentration of oxygen in automotive exhaust gases is shown in partial cross section in FIG. 1. The device includes four components: an oxygen-ion-conductive solid electrolyte 1, electrodes 2 and 3 made of an electroconductive material, and a protective layer 4. The solid electrolyte 1 is positioned between the atmosphere A and the exhaust gas or emission E and generates an electromotive force in proportion to the differential oxygen concentration between the atmosphere and the exhaust gas. The electrode 2 is positioned on the inner surface of the solid electrolyte 1 with the electrode 3 on its outer surface. Both electrodes act as oxidation catalysts. The protective layer 4 is used to protect the electrode 2 from the physical and chemically corrosive effects of the exhaust gas.
Two examples of such a conventional device are disclosed in Japanese Unexamined Published Patent Applications Nos. 89686/1979 and 13828/1980. The devices described in these documents use a double- or triple-layered structure for the protective layer 4. However, because of the relative thinness of this protective layer 4 (typically less than about 100 microns), lead cannot be trapped in a sufficient amount to provide the desired resistance to lead contamination.
Japanese Unexamined Published Patent Applications Nos. 90294/1973 and 53456/1981 disclose other examples of conventional oxygen detectors. In order to provide an enhanced resistance to lead contamination, the protective layer 4 used in these examples is deposited with a lead-trapping metal through dipping process. However, depending on the dipping method employed, the lead-entrapping metal is mostly supported on the outer surface of the protective layer 4 and lead has a tendency to be deposited only in that area, thus causing occasional clogging that may eventually lead to a lowered responsiveness of the detector. The protective layer 4 is also equipped with a lead-resistant catalyst layer, but if the concentration of the catalyst is increased with a view to enhancing the resistance to lead contamination, the responsiveness of the detector is decreased. A lowered responsiveness is also caused by using an excessively thick catalyst layer.
Japanese Patent Publication No. 18146/1982 discloses a detector that employs an electrode coated with a protective layer 4 which is formed by first applying a slip containing a heat-resistive metal oxide and a catalytic material, drying the slip, and then sintering the dried slip. The biggest problem with this type of detector lies in the fact that it is difficult to prepare a slip having an optimum porosity. An excessively small porosity will prevent the entrance of gases into the detector, and if the porosity is too great, the protective layer 4 may be corroded by a reducing exhaust gas, possibly resulting in an open-circuit failure in the electrode 2. Additionally, the slip has a tendency to not firmly adhere to the electrode 2 and may easily come off during service.